1. Field of the Invention
The present invention relates to adhesive compositions comprising an .alpha.-cyanoacrylate compound as a main component thereof and a fine particulate silica as a thixotropic agent.
2. Description of the Prior Art
Adhesive compositions comprising an .alpha.-cyanoacrylate compound as the main component thereof have found wide use as instantaneous adhesives. Examples of .alpha.-cyanoacrylate compounds for use in such compositions are methyl-.alpha.-cyanoacrylate, ethyl-.alpha.-cyanoacrylate, isobutyl-.alpha.-cyanoacrylate, n-butyl-.alpha.-cyanoacrylate, n-propyl-.alpha.-cyanoacrylate and similar alkyl-.alpha.-cyanoacrylates, allyl-.alpha.-cyanoacrylate, propargyl-.alpha.-cyanoacrylate and similar .alpha.-cyanoacryalates having an unsaturated group, 2-methoxyethyl-.alpha.-cyanoacrylate, 2-ethoxyethyl-.alpha.-cyanoacrylate and similar .alpha.-cyanoacrylates having an alkoxyl group and alkyloxyalkyl-.alpha.-cyanoacrylates, tetra-hydrofurfuryl-.alpha.-cyanoacrylate and similar special .alpha.-cyanoacrylates.
These .alpha.-cyanoacrylates have heretofore been industrially produced usually by the following process.
A condensation product is prepared by reacting a cyanoacetate with formaldehyde or a polymer thereof in an inert organic solvent in the presence of a basic catalyst, the resulting water is distilled off in the form of an azeotropic mixture with the solvent, the solvent is then distilled off, the product is thereafter subjected to thermal depolymerization in a vacuum in the presence of phosphorus pentoxide or like nonvolatile acid to obtain a crude .alpha.-cyanoacrylate, and the crude .alpha.-cyanoacrylate is purified by distillation.
The basic catalyst used in this process is usually a nonpolymerizable basic compound such as piperidine or similar amine, caustic soda, caustic potash, alkoxide of alkali metal or the like. The basic catalyst conventionally used is a compound which will not polymerize even if subjected to the action of acids and heat.
.alpha.-Cyanoacrylate compounds are used for instantaneous adhesives, which include one containing fine particulate silica to impart thixotropic properties to the adhesive.
However, the .alpha.-cyanoacrylate adhesive containing the fine particulate silica has the serious drawback of being low in stability.
Accordingly, a fine particulate silica has been developed which has its surface modified. More specifically, the hydrophilic hydroxyl on the surface of fine particulate silica is rendered hydrophobic or oleophilic, for example, by treatment with polydimethylsiloxane, trialkoxyalkylsilane or the like.
We have conducted continued research on .alpha.-cyanoacrylate adhesives of the type mentioned and conceived the novel idea of using a surface-untreated fine particulate silica as a thixotropic agent to eliminate the cumbersome procedure of surface-treating fine particulate silica before use and to develop .alpha.-cyanoacrylate adhesive having high stability.
On the other hand, we have conducted research also on processes for preparing .alpha.-cyanoacrylate compounds and consequently developed a process for preparing .alpha.-cyanoacrylate compounds which are well-suited for use in adhesives as a main component thereof. This process has been patented as U.S. Pat. No. 4,764,545.